Serial communication links that employ channels that exhibit low pass filter effects often use transmit pre-emphasis, receiver equalization, or a combination of the two to overcome the loss of high-frequency signal components. Adaptive transmit pre-emphasis or receive equalization may be used for marginal links or links whose transfer characteristic change over time. In either case, the received signal quality may be measured at the receiver. Adaptive transmit pre-emphasis schemes may therefore use some form of back-channel. communication to relay indicia of signal quality back to the transmitter. Unfortunately, the need for a backchannel renders the design and implementation of adaptive pre-emphasis challenging and complex. Also important, some integrated circuits that receive data via a serial link may not include a compatible backchannel receiver with which to communicate. The transmit and receive circuitry may be parts of integrated circuits from different vendors, for example, in which case the two vendors would have to agree in advance upon a backchannel communication scheme and design their circuitry accordingly. Such collaboration may be impractical.
Adaptive receive equalization does not require backchannel communication, and thus avoids many of the problems inherent in adaptive transmit pre-emphasis. Optimum pre-emphasis and equalization settings are data specific, however, because different data patterns have different spectral content, and thus are affected differently by low-pass characteristics of the channel. As a first-order approximation, the higher the frequency, the greater the attenuation. Transmitters “know” the transmitted data pattern in advance, and thus can tailor the transmit pre-emphasis to the data; in contrast, receivers do not know the received data pattern in advance, so adaptive equalization that addresses changes to the incoming data is much more difficult.
Some adaptive receive equalization schemes measure the power density of received signals at two frequencies and adjust the receive equalizer to maintain some desired ratio of the two power densities. Unfortunately, such schemes may not provide appropriate levels of equalization for frequencies other than those monitored. Furthermore, noise at a monitored frequency contributes to the measured power density, and consequently results in erroneous equalizer settings. There is therefore a need for receive equalization systems and methods that are more responsive to received data patterns and less sensitive to noise.